from your work I guess you are pulling at 36.87 deg up and not pushing down. If so I agree with the 4.84 m/s^2
now the sum of forces on the upper package must give an acceleration of 4.84 m/s^2
20 (4.84) = 400 cos 36.87 - [200 -400 sin 36.87] mu
hmmm, you must be pulling up on the lower package, because 400 sin 36.87 would lift the upper one right off. In that case
20*4.84 = 200 mu
mu = .484
There are two things you did not tell me so I had to assume.
1. You are pulling up, not pushing down.
2. You must be pulling up on the lower package or the top one would lift off.
If these assumptions are incorrect, you must change signs all over the place.